Various types of rotating machinery include components (such as blades or buckets) that rotate within a space defined by a stationary component or shell. In such machinery it may be of interest, for various operational reasons, to monitor the spacing or clearance maintained between the rotating and stationary components. For example, a steam turbine has a rotating bucket that is disposed adjacent a carrier. The clearance between the rotating bucket and the carrier varies due to various operating conditions, such as changes in temperature, oxidation of the bucket tip, and so forth. It is desirable that a gap or clearance between the rotating bucket and the carrier be maintained during operation of the steam turbine.
Typically turbo machinery has thermal growth that occurs on both the rotating and stationary components, This expansion may occur in both the axial and radial directions and typically it is desired to minimize the clearance (such as the radial clearance) between the rotating components and the housing (e.g., the shell). However, design and operation changes may affect determination of this clearance measurement. One approach employed to determine minimum clearance values is via a soft rub pin with measurements taken during a shut down period, i.e., off-line. Such off-line measurement may be unsuitable for understanding and optimizing clearance in a running turbine system.